Viscosity is the measurement of a fluid's resistance to flow. Viscosity is an important property that is commonly measured and/or controlled with respect to fluids employed in the printing and coating industries. One current method to measure viscosity is to measure the time it takes for a volume of fluid to flow out of a cup with a specified diameter hole in the bottom of the cup. This type of cup is generically referred to as an efflux cup. The time measurement, in seconds, is converted into units called centistokes (cSt).
Centistokes are defined by multiplying the viscosity (in centipoise) by the specific gravity of a fluid. Thus, viscosity (in centipoise) can be calculated by dividing centistokes by the specific gravity of the fluid. Although indirect, this calculation provides a quick and inexpensive way to measure the complex property of fluid viscosity.
Efflux cups are available in various shapes and sizes, and are widely available. Common types of efflux cups include Zahn, Shell and Ford cups. The most common type of efflux cup used commercially today is the ZAHN cup. ZAHN cups are commercially available in different size ranges, denoted 1–5, and are differentiated by the diameter of the bottom drain or orifice.
Because the ZAHN cup is so common and widely used, the term “Zahn” has become an eponym. In this regard, the experimental units of time reported for drainage of fluids from a ZAHN cup are referred to as Zahn-seconds. Today it is common to report viscosity simply as X number of Y Zahn-seconds (for example, 15.5 #2 Zahn-seconds), where X is the drainage time measured in seconds, and Y is the number of the ZAHN cup. Similarly, one can state units as Shell seconds when using a Shell cup or Ford seconds when using a Ford cup, for example. Rarely do users convert these times into centistokes or centipoise.
The drainage times of fluids draining from efflux cups can be standardized by measuring the time between a starting time and a stop time. For example, an efflux cup is placed into and removed from a fluid. The starting time occurs as soon as the top rim of the efflux cup breaks the surface of the fluid upon removal. The fluid then proceeds to drain from the efflux cup through the orifice in the bottom of the cup. The stop time is defined as the instant the stream of fluid breaks between 1 and 2 inches below the efflux cup. That is, the stop time occurs when a continuous stream of fluid draining from the hole in the efflux cup becomes non-continuous between 1 and 2 inches below the efflux cup.
Currently, the drainage times are measured by manually starting and stopping a stopwatch after visual interpretation of the start and stop times. Problems with this approach are obvious: operator reaction time and varying interpretations of start and stop times. These problems lessen the accuracy and precision of the viscosity measurement.
Proposed solutions to these accuracy and precision problems include using floats, levers and even optical emitter/detector pairs to determine the start and stop times. However, many times the fluid being measured can remain on the float, lever or optical emitter/detector pair and interfere with the measurement of either the stop or start time. That is, the fluid can coat the device and interfere with the device's measurements. For example, ink that remains on an optical detector can cause inaccurate or false measurements of the start or stop times.
One solution that has yet to be employed is the use of electric current and the ability of the fluid to conduct the electric current to more accurately and precisely measure the drainage start and stop times. That is, a new solution is to utilize the electric conductance of a fluid to measure the start and stop times.
A device, capable of being added to any conventional viscosity measuring cup, and which could accurately determine start and stop times would virtually eliminate human error associated with conventional viscosity measurements. This, along with simplicity and speed, is one of the main goals in automating viscosity-testing instrumentation.